How Efficient Are Solar Panels? 

People often get confused when talking about the “efficiency” of solar panels.

Since “efficient” and “efficiency” are commonly used words, it’s easy to overlook that PV panel efficiency is a highly specific metric.

Solar panel systems are an extraordinarily efficient way to harness clean, renewable electricity for your home.

But when it comes to measuring efficiency, not all photovoltaic (PV) panels are created equal.

If you’re in the market for solar panels, understanding the efficiency specification is essential for making an informed purchase decision.

So, what exactly is solar panel efficiency? 

Read on to find out.  

(Source: Energy Education)

What Is Solar Panel Efficiency? 

Solar panel efficiency measures the percentage of sunlight hitting the photovoltaic surface of a solar panel that gets converted into direct current (DC) electricity. 

All currently available PV modules (such as solar panels) which convert sunlight into electricity use solar cells to achieve the photovoltaic effect.

How Is Solar Panel Efficiency Measured? 

Solar panel efficiency is determined in a laboratory under strict Standard Test Conditions.

Condition Type	Standard Test Condition	Real-World Conditions
Solar Incident Angle	Always zero, irradiation beam always normal to the PV panel*	Variable and depends on time, date, and site latitude. In the case of rooftop systems, roof orientation, and inclination govern system capacity.
Solar Irradiation	Always equal to 1000 Watts/m²*	Variable and depends on the time, date, and site latitude. Limited sunshine hours bound system capacity.
Ambient Temperature	Always 25°C*	Variable and depends on the time, date, weather condition, and site latitude. Higher ambient temperature degrades PV panel efficiency and reduces system output.
Air Mass Coefficient (AM)	Always equal to 1.5*	Variable and depends on the time, date, and site latitude. Higher AM with higher latitudes.
System Losses (e.g., Wiring, Inverter)	Always Zero*	Variable and depends on the design and location of PV panels, inverter, and grid meter.

*Cannot be achieved in real operation (Source: ResearchGate)

As stated in the table, real-world conditions rarely (if ever) match Standard Test Conditions.

Environmental variables are a primary reason solar panels don’t always output their full-rated power. Rated power is determined under similar conditions. 

Standardization is a good thing for consumers. If all reputable manufacturers follow the same Standard Test Conditions, you can make a more informed opinion about the quality of the product.

Newcomers to solar power often make the mistake of thinking that 4 x 400W rigid solar panels will produce 1600W of electricity per hour for every hour of daylight.

While you may achieve that level of production during peak sunlight hours, it’s essential to remember that rated power indicates the MAXIMUM output of a solar panel. You’ll never generate more. You’ll typically generate less. 

Assuming 75% of rated power per hour of sunlight per PV panel with an efficiency rating of 20% or higher is a solid place to start when estimating how many modules you’ll need to meet your requirements.

Why Is Solar Panel Efficiency Important?

Solar panel efficiency is essential to planning a PV panel array that meets your electricity generation goals.

Efficiency (as a spec) determines how many PV panels you must purchase and how much surface area you require to install them.

For example, EcoFlow mountable solar panels have an industry-leading efficiency rating of 23%.  The EcoFlow 400W rigid solar panel has dimensions of 67.8 × 44.6 × 1.38 in (172.2 × 113.4 × 3.5 cm) for a footprint of 1.94m².

A 400W solar panel from another manufacturer with an 18% efficiency rating has a footprint of 2.42m^2.

In this example, the surface area of a single PV panel with a 5% lower efficiency rating will take 25% more space on your roof. 

For context, if you were to install 4 x EcoFlow 400W rigid solar panels for connection to the EcoFlow DELTA Pro, the approximate surface area of the installation would be about 10m². Ideally, the installation area should be in direct sunlight and unobstructed by structures or trees that might cause shade. 

Also, bear in mind that this estimate includes the recommended 20 – 30 cm distance between adjacent panels. 

The surface area required for 4 x 400W solar panels with 18% efficiency would be approximately 12.5m² to generate the same amount of electricity.

Most homes have limited roof space that receives optimal sunlight at all times of day. If you’re lucky enough to have excess surface area suitable for installation, efficiency may not be as pressing a concern.

However, lower efficiency can also impact the number of PV panels you need to purchase and can significantly lengthen your solar payback period. 

Which Types of Solar Panels are the Most Efficient? 

There are currently three types of PV panels commonly used in residential solar power installations. 

Monocrystalline, polycrystalline, and thin-film.

Each varies significantly in efficiency. 

Here are the average efficiency ratings for each, along with a brief explanation below.

Solar Panel Type	Efficiency (%)
Monocrystalline	18-23%
Polycrystalline	15-17%
Thin-film	7-18%

Monocrystalline Solar Panels

Monocrystalline solar panels have long set the benchmark for efficiency and aesthetics.

The photovoltaic cells in a monocrystalline solar panel consist of a single, pure lab-grown silicon crystal. 

Rigid monocrystalline solar panels are made up of numerous solar cells — the number per PV panel varies based on the module’s rated power and the efficiency of the individual cells.  

The silicon solar cells are shielded from the elements by a transparent material like tempered glass and encased in a metal frame (typically aluminum or stainless steel).

The purity of monocrystalline solar cells gives them their distinctive all-black look, which helps preserve the aesthetic value of your home. 

Purity also makes them significantly more efficient than polycrystalline silicon solar cells. 

Polycrystalline solar cells are comprised of multiple fragments of silicon, creating current-resistant boundaries that waste electricity and reduce performance. 

Due to their construction, these are the most efficient solar panels for residential solar power systems. 

Unfortunately, the manufacturing process for monocrystalline solar panels is more resource-intensive and results in more wastage of materials like silicon. For this reason, the price tag on monocrystalline solar panels is typically higher than the other two types. 

But the efficiency gain should more than make that up in the long run.

On average, monocrystalline PV panels have efficiency ratings of around efficiencies of around 20%. 

EcoFlow’s mountable and portable solar panels boast an industry-leading efficiency of ~23%.

(Source: ASES)

Polycrystalline Solar Panels

Polycrystalline solar cells are created by melting fragments of silicon and pouring the liquid into a square mold. Because all of the silicon used in the manufacturing process can be recovered, there is little, if any, wastage.

Polycrystalline solar cells come out of the mold in their final shape as square wafers. Unlike with mono cells, no further cutting or trimming is required. The square shape gives polycrystalline their distinctive checkerboard look. Impurities introduced in the process turn them into granular shades of blue.

Polycrystalline solar panels tend to be less costly upfront at the expense of efficiency and aesthetics. Poly panels typically offer 15-18% efficiency ratings.

Thin-Film Solar Panels

Thin-film solar panels can consist of several different materials:

• Amorphous silicon (a-Si) panels: 7% efficiency
• Cadmium telluride (CdTe) panels: 9% to 15% efficiency
• Copper Indium Gallium Selenide(CIGS) panels: 12% to 14% efficiency

Thin-film solar panels utilize very thin layers of photovoltaic material and are more flexible than other types.

However, they are also the least efficient solar panels, with an average overall rating of about 12%.

What Impacts Solar Panel Efficiency? 

A solar panel’s efficiency rating is determined in a laboratory under Standard Test Conditions. 

Technically, there are only a few factors that impact the potential of a solar cell to capture a finite amount of electricity from the same level of direct sunlight (solar irradiation)

For example, neither a monocrystalline solar panel with a 23% efficiency rating nor a thin film panel with 8% efficiency captures solar power at night. The efficiency rating achieved in the lab doesn’t change — there’s simply no sunlight to harvest.   

Several factors do impact PV module efficiency.

But it’s essential to bear in mind that efficiency is a metric that’s related to — but distinct from — output.  

Here are the factors that impact solar panel efficiency, followed by external conditions that affect efficiency and/or electricity production. 

Solar Cell Structure and Composition

The composition of photovoltaic materials in a solar panel is the most significant determinant of its efficiency rating. As covered above, the most efficient solar cells are made from monocrystalline silicon, followed by polycrystalline, then thin-film. 

Temperature

Solar panels are most efficient between 59 and 95 degrees Fahrenheit (15°C – 35°C). 

Extremely low or high temperatures will reduce the efficiency of your solar array. 

If you’re wondering whether PV panels still work in winter, the good news is that efficiency actually improves in colder temperatures. 

The photovoltaic effect captures solar energy from light — not heat. In scorching environments that receive many hours of peak sunlight — like a desert — solar panels actually need to be cooled to perform optimally.  

PV Panel Age and Declining Efficiency Over Time

High-quality PV panels typically last over 25 years without requiring replacement or significant maintenance. 

However, solar panel efficiency decreases slowly over time. 

NREL research shows that PV panels have a median degradation rate of around 0.5% per year or slightly more for rooftop systems or hotter climates. 

Despite degrading over time, most solar panels will continue to produce at least 80% of their output capacity after 25 years.

Maintenance

Solar panels are remarkably resilient. 

Even in harsh climates, PV modules require very little maintenance. However, a thorough inspection and cleaning every six months or as needed can extend the lifespan of solar panels and ensure they operate at maximum efficiency.

Soiling

Soiling refers to dirt and debris that accumulates on the photovoltaic surface of solar panels. Soiling and the following factors don’t technically change a PV module’s efficiency rating. 

However, outside of a lab, real-world conditions DO impact a solar panel’s capacity to make the most of available sunlight, thus impacting electricity production and practical efficiency.

As you can imagine, as dirt accumulates on the tempered glass (or other surface) that protects the solar cells, less sunlight makes its way to the photovoltaic cells underneath. 

Think of solar panels as windows. The dirtier they are, the less light gets in. In the case of PV modules, dirt and debris decrease production and practical efficiency. 

Ice and Snow

If you live in a location where temperatures dip below zero, you’re likely to get periods of icy rain and snow. 

In the same way that dirt and debris can impede the ability of solar cells to function optimally, snow can be even worse. In some cases, it may blot out the sun altogether.

Depending on the tilt of your panels, snow may slide off without your intervention. But if you see a significant dip in electricity production after a blizzard, you should manually remove ice and snow from your panels.

Panel Orientation and Tilt

Optimizing the panel’s angle and position relative to the sun affects its operating efficiency significantly. You want to position it to receive the maximum direct sunlight available during the day. 

In applications where roof design limits optimal orientation and tilt, portable solar panels may be better able to take advantage of peak sun hours. 

Solar trackers can also be a viable — if pricey — option in some locations.

Shade

Objects and structures that regularly impede sunlight capture substantially reduce practical efficiency and electricity production. 

Trees and neighboring buildings are common culprits. You should work to clear obstructions when possible. For example, trimming branches regularly to keep the sunlight’s path clear. 

The amount of shade obstructions cast varies throughout the day. 

The best way to deal with shading is to observe its patterns before you mount your panels. Ideally, position them where there’s no shade. 

Failing that, mount the panels where they receive the least shade during peak sunlight hours. If you can’t avoid obstructions, position the panels where they’re shaded at the beginning or end of the day, when solar irradiance is diminished.

Availability of Direct Sunlight

It should go without saying, but the amount of electricity your solar panels can produce depends on the amount — and strength — of solar irradiance (sunlight). 

Solar panels don’t work at night, and production will be diminished on cloudy days.

Inverter and Balance of System (BoS) Efficiency

Solar panels don’t operate in isolation.

Solar components other than photovoltaic modules are commonly called the balance of system. 

Depending on whether your residential solar power solution is on-grid or off-grid, the following components may be optional or required.

• Solar inverter
• Solar battery
• Storage inverter
• PWM or MPPT charge controller
• Battery management system
• Cables and wiring
• Mounting hardware

A residential balance of system solution like EcoFlow DELTA Pro Ultra includes all of the components required to operate up to 42 x EcoFlow 400W rigid solar panels. 

You can also buy BoS components separately from different manufacturers. Just ensure cross-compatibility.

Along the journey that DC electricity makes from your solar panel array to your home circuit panel, bidirectional meter, or portable power station, there are many potential ways to lose power along the way.  

The solar inverter is often the culprit for reduced efficiency in your BoS, but lead acid solar batteries and PWM charge controllers can also be problematic. 

Cables, wiring, and the distance between your solar panels and other components are also factors. The longer distance electricity travels, the more is lost along the way. 

If you want to have the most efficient (in both senses of the word) solar panels and balance of system, think long-term when making a purchase. Beware of module mismatch and conversion losses from low-quality inverters.

You’ll get a better return on investment that way. 

Do Extreme Conditions Affect the Efficiency of Solar Panels? 

Temperature, especially extreme heat, is detrimental to solar panel efficiency. Cold, by comparison, has to be very cold before a negative impact on performance.

But what about other frequent extreme weather events?

Here’s a summary. 

Snow

Snow accumulation on solar panels can completely block out sunlight, effectively decreasing a PV module’s operating efficiency to zero.

If you live in a location where snow is a regular occurrence, you should consider tilting your solar panels at an angle that will help snow glide off after a storm.

But not at the expense of optimizing year-round exposure to peak sunlight.

If you have a particularly heavy blizzard — or notice decreased performance from your system after a snowfall — it’s time to pull out the ladder and clean your panels manually.

Better yet, hire a professional to do it and save yourself the risk of slipping and falling on an icy surface.

Ice

Like snow, ice can build up on the surface of your solar panels under certain conditions. 

Unlike snow, ice is mostly transparent and shouldn’t reduce efficiency nearly as much as snow. However, it’s wise to carefully clear ice build-up from your PV panel array if it becomes substantial.

DO NOT use a scraper that you would use on a car windshield on your solar panels. You may end up damaging the protective surface above the solar cells, causing more permanent damage than the temporary effects of leaving ice where it is.

Specialized tools like a foam-headed snow broom or a household broom with soft bristles should be more than enough to do the job safely.

If you live in a climate where snow and ice make solar panel maintenance a yearly struggle, there may be hope.

Researchers are hard at work on snow and ice-resistant and repellent coatings that should make their way to market relatively soon.

Hail

Solar panels are built to withstand a lot of punishment and are designed to survive the elements year-round.

Thanks to materials like tempered glass, most PV panels will be unharmed even by a heavy hail storm. For example, EcoFlow’s rigid solar panels have an IP68 water/dustproof rating and can take a massive beating.

However, if you notice a significant dip in performance after a hailstorm, it’s best to inspect your panels for signs of damage. 

Inspection is essential if your solar panels are connected in series instead of parallel. One damaged panel will diminish the production of the entire array.

UV Degradation

UV-induced degradation can delaminate and discolor materials in solar panels and cells within a PV panel. 

Ironically, recent research has shown that modern high-efficiency solar panels may be more susceptible to UV degradation than older PV technology.

However, the gains in efficiency and output more than makeup for any increased susceptibility to UV degradation. 

As with virtually everything else, the slow deterioration of PV panels over time is inevitable. 

High-quality solar panels typically still function at 70-80% of their original output after 30 years.   

Frequently Asked Questions

Final Thoughts

Now that you understand the relationship — and differences — between “efficiency” and solar panel efficiency rating, you’re in a much better position to make an informed purchase decision on a PV panel array that meets your needs. 

At EcoFlow, we offer rigid, portable, and flexible solar panels in rated power wattages of 100W – 400W and options in between.

All of EcoFlow’s solar panels utilize monocrystalline solar cells with industry-leading efficiency ratings of 22-23%

Check out our selection today.